Chlamydia trachomatis is a common sexually transmitted pathogen which causes trachoma, conjunctivitis, pneumonia of the newborn and genital infections. We have recently demonstrated that C. trachomatis may cause proctitis in homosexual men, and our recent screening studies have demonstrated positive rectal cultures for C. trachomatis in 24 (8.3%) of 288 randomly selected homosexual men and from 33 (21%) of 155 heterosexual women. Concomittant cervical infection was demonstrated in 60% of the women with rectal infections. Ninety-three percent of the infected men and 72% of the infected women had signs or symptoms of proctitis. Five isolates were LGV strains of C. trachomatis and were indistinguishable clinically and histopathologically from other forms of granulomatous colitis. Non-LGV strains were associated with mild inflammatory proctitis. All chlamydial infections were associated with the rise in antibody titer with geometric mean titers of 1:1,512 and 1:282 in LGV and non-LGV infections. In summary, these studies have demonstrated that C. trachomatis may induce rectal infection which may be associated with a wide range of clinical manifestations ranging from nonspecific proctitis to severe granulomatous colitis. In order to facilitate diagnosis and to assess for the presence of Chlamydia in pathologic tissue from AIDS and from other forms of inflammtory bowel disease, we developed an assay utilizing monoclonal antibody directed to C. trachomatis antigen which appears capable of detecting elementary bodies on cervical, urethral, and rectal swabs, and in pathologic material. Sensitivity and specificity of this assay is 97% and 98% respectively. Further studies are in progress to detect chlamydial antigen in pathologic material from AIDS patients and patients with genital and rectal infections by immunofluorescence. Studies on immunopathogenesis of chlamydial infection will be initially carried out in our recently developed experimental non-human primate models of C. trachomatis salpingitis and proctitis.